The present invention relates to liquid development of electrostatic latent images, and more particularly relates to means for achieving a uniform development zone between a liquid developer applicator and an imaging member.
In electrostatographic copying machines, the development of the latent image on the photoreceptor advantageously is carried out by providing a developer fluid in operative engagement with an imaging member such as a photoreceptor whereby the liquid developer is attracted to the imaging member or photoreceptor in the charged areas. Subsequently, the photoreceptor is brought into contact with an interposing material, e.g., sheet material such as paper so that the developed images are transferred thereto to produce the desired copy. Further particulars of the general concept of liquid development in copying machines may be found in U.S. Pat. No. 3,084,043, for example.
In a preferred prior art electrostatic latent image development mode, liquid developer may be advantageously brought into operative contact with the imaging member by means of an applicator roll that rolls over the charged imaging member. A typical example of such an arrangement is shown in U.S. Pat. Nos. 3,084,043 and 3,806,354 which describes electrostatographic copying apparatuses and methods wherein the applicator is a rigid cylindrical member having on its surface a pattern of grooves and ridges which comprise lands and valleys, respectively. A liquid developer is maintained in the valleys below the surface of the lands. The applicator is positioned to come into contact with a photoreceptor bearing on its surface an electrostatic latent image. In a typical electrostatographic copying apparatus the imaging member or photoreceptor is also a cylindrical member comprising a conductive substrate and a photoconductive coating which supports the electrostatic latent image. The electrostatic latent image is produced by first charging the entire surface of the imaging member or photoreceptor in the dark and then by exposing the charged surface to imagewise radiation. The portions of the charged photoreceptor surface which are struck by the radiation are discharged, leaving an image pattern of charged on the photoreceptor surface in the non-radiation-struck areas.
The photoreceptor surface bearing the electrostatic latent image and the applicator are brought into moving contact during which the liquid developer is drawn to the photoreceptor from the valleys of the applicator roller by the charges which form the electrostatic latent image. Typically, the image is then transferred to an image receiving member such as an interposer web or paper by pressure contact between the photoreceptor and a roller.
Although both of the surfaces may be flat, it is more common for at least one of the surfaces to be arcuate to facilitate the moving of the applicator past sequential points on the photoreceptor while the two are in contact. In compact electrostatographic copying devices the surfaces are small diameter cylinders to facilitate the cooperative movement of the surfaces in a confined space. Such movement typically occurs at speeds of about 4 inches per second, although moving contact resulting in the transfer of liquid developer from the applicator to the photoreceptor occurs at speeds ranging generally from about 2 to about 70 inches per second.
To assure proper development of a uniform nip width or development zone must be provided across the area of contact between the developer applicator roll and the photoreceptor during their movement together to provide a substantially uniform length of time and space in the areas of contact during which the liquid developer is able to move from the applicator valleys to the photoreceptor surface. If this nip width or development zone is uniform, substantially uniform amounts of liquid developer will be transferred to the photoreceptor in response to substantially equally charged portions of the image. Most of the prior art development processes and devices do not provide means for maintaining this desirably uniform nip width or development zone.
Movement of a brush developing station toward and away from a photoreceptive drum assembly whereby the inspection, repairs and maintenance of the developing station are facilitated, has been proposed in British Pat. No. 1,317,289. In that disclosure however, the means for providing movement of the brush developing station are not adequate for maintaining a uniform nip or development zone between a liquid developer applicator roll and an imaging member.